present invention relates to shock sensors in general and shock sensors employing reed switches in particular.
Reed switches have found wide use in shock sensors, particularly as safing sensors in automobiles. Typically automobile crash sensing is performed by integrated micro device sensors which are incorporated onto chips which assess the magnitude and direction of the crash and employ preprogrammed logic to decide whether and how to deploy or activate various safety systems. These systems include air bags and seat belt retractors. Such micro sensors can be very cost-effectively incorporated into a safety system logic. However, such small scale devices are subject to electromagnetic interference and related phenomenon giving rise to possible false sensor outputs.
The need for a macro scale sensor arises to provide a safing sensor which provides the programmed logic with an indication that a crash of sufficient magnitude to warrant deployment of safety systems is in fact occurring. Shock sensors employing reed switches meet the need for a large scale device while at that the same time allowing a relatively small sized package which can be directly mounted onto a circuit board. A reed switch is resistant to electromagnetic interference and the hermetic seal formed by the glass capsule about the reeds results in a highly reliable switch which is sealed from the atmosphere. Thus, reed switch based shock sensors are usually the design choice for safing sensors forming part of a vehicle safety system.
Reed switch based shock sensors have been designed with multiple axes of sensitivity, yet such devices are typically considerably more expensive than unidirectional shock sensors or are more sensitive to large scale vibration. A typical reed switch based shock sensor has an acceleration sensing magnetic mass which is held against a stop by a spring. The spring is typically pre-loaded so that no motion of the sensing mass takes place unless the acceleration loads exceed a selected value. Obtaining pre-loaded sensing masses in a bidirectional shock sensor has proven problematic.